This invention relates to a photomultiplier employed in a scintillation detector to detect radiation such as gamma rays, and more particularly to a photomultiplier for detecting the incident position of radiation.
There has been conventionally known a photomultiplier used in a scintillation detector to detect the incident position of radiation such as gamma rays.
FIG. 1(A) is a sectional front view showing a photomultiplier employed in a conventional scintillation detector, and scintillators combined suitably with the photomultiplier to emit light in response to the incidence of radiation such as gamma rays. FIG. 1 (B) is a sectional side view of the scintillation detector shown in FIG. 1(A). As shown in FIGS. 1(A) and 1(B), the scintillators 101 and 102 and the photomultiplier 103 constitute the scintillation detector 100.
The scintillators 101 and 102 are made of light emitting material such as bismuth germanium oxide (Bi.sub.4 Ge.sub.3 0.sub.12). When radiation such as gamma rays are applied to the scintillators 101 and 102, the latters 101 and 102 emit light beams 420 nm (nano-meters) in wavelength. Each of the light beams thus emitted is converted into an electrical signal by the photomultiplier 103 which is so positioned as to receive the light beams. The position determination of the incident beam to the scintillators 101 and 102 is performed by detecting which of the anodes OT, and OT.sub.2 of the photomultiplier 103 outputs a pulse current.
The photomultiplier 103 has two photocathodes 104 and 105 which face the two scintillators 101 and 102, respectively, thereby determining which of the scintillators 101 and 102 has received the radiation. The photocathodes 104 and 105 are provided on inner surfaces 108 and 109 of a transparent end face plate 107, respectively, which forms the bottom of a rectangular cylinder-shaped air-tight tube 106.
The photomultiplier 103 has two focusing electrodes 110 and 111, two arrays of dynodes 112 through 118 and 120 through 126, and two mesh anode electrodes 127 and 128 in correspondence to the two photocathodes 104 and 105. Therefore, upon reception of light, the photocathode 104 emits photoelectrons, which are multiplied by means of the dynodes 112 through 118 and output through the anode electrode 127. Similarly, upon reception of light, the photocathode 105 emits photoelectrons, which are multiplied by means of the dynodes 120 through 126 and outputted through the anode electrode 128.
The focusing electrodes 110 and 111 are used to positively introduce the photoelectrons from the photocathodes 104 and 105 to the respective arrays of dynodes 112 through 118 and 120 through 126. The focusing electrodes 110 and 111 have parts 129 and 130 adjacent to each other, respectively. The part 129 serves as a partition wall for preventing the photoelectrons emitted from the photocathode 104 from being applied to the arrays of dynodes 120 through 126, and similarly the part 130 serves as a partition wall for preventing the photoelectrons emitted from the photocathode 105 from being applied to the array of dynodes 112 through 118.
The dynodes 112 through 118, and 120 through 126 are curved as required and supported on electrical insulation supporting members 131 and 132.
As shown in FIG. 1(A), the sections of the inner surfaces 108 and 109 of the end face plate 107 which is perpendicular to the longitudinal direction of the dynodes 112 through 118, and 120 through 126 (i.e., perpendicular to the surface of the drawing the FIG. 1(A), and accordingly the sections of the photocathodes 104 and 105 have a predetermined curvature (a radius of curvature R1), and have the centers on the central axes A--A and B--B of the focusing electrodes 110 and 111, respectively. Similarly, as shown in FIG. 1(B), the sections of the inner surfaces 108 and 109 of the end face plate 107, which is perpendicular to the longitudinal direction of the dynodes 112 through 118, and 120 through 126, and accordingly the sections of the photocathodes 104 and 105 have a predetermined curvature (a radius of curvature R2) and have their centers of curvature on the central axes A--A and B--B of the focusing electrodes 110 and 111, respectively.
In the scintillation detector 100 containing the photomultiplier 103 thus constructed, a gamma ray .gamma.1 is incident to the scintillator 101 to emit scintillation light. Of the light thus emitted, light beams advancing along optical paths are typically designated by e11 and e12 as shown in FIG. 1(A) respectively. The light beam e11 is incident directly to the photocathode 104 of the photomultiplier 103 to emit a photoelectron P11 therefrom. On the other hand, the light beam e12, after being reflected by the side wall of the scintillator 101, is incident to the photocathode 104 of the photomultiplier 103 to emit a photoelectron P12 therefrom.
The photoelectrons P11 and P12 emitted from the photocathode 104, being focused owing to the configuration in section (R1 and R2) of the photocathode 104 and by the focusing electrode 110, are applied to the first dynode 112. The photoelectrons are multiplied by the dynodes 112 through 118, thus being outputted as a pulse current through the output terminal OT1 of the anode electrode 127.
The pulse currents provided at the output terminals OT1 and OT2 of the anode electrodes 127 and 128 are applied to a pulse counter (not shown), so that the number of pulses corresponding to the gamma rays .gamma.1 (or .gamma.2) incident to the scintillator 101 (or 102) can be detected. That is, the pulse counter is used to detect how many pulse currents are supplied through either of the output terminals OT.sub.1 and OT.sub.2, thereby to determine how many gamma rays are incident to either of the scintillators 101 and 102.
In the conventional photomultiplier 103, as shown in FIGS. 1(A) and 1(B), the wall 129 of the focusing electrode 110 prevents the photoelectrons emitted from the photocathode 104 on one side from being applied to the first dynode 120 on the other side, and similarly the wall 130 of the focusing electrode 111 prevents the photoelectrons emitted from the photocathode 105 on the other side from being applied to the first dynode 112 on the one side. However, the conventional photomultiplier is disadvantageous for the following reasons: The two inner surfaces 108 and 109 of the end face plate 107 have the predetermined radius of curvature and are adjacent to each other, so that the plate 107 is larger in thickness at the border between the two photocathodes 104 and 105. Therefore, a part of the light emitted in the scintillator on one side (for instance 101) may advance towards the photocathode on the other side (for instance 105) instead of the photocathode 104 when passing near the border between the two inner surfaces 108 and 109 of the end face plate 107. That is, so-called "light mixing" occurs in the photomultiplier, as a result of which the incident position is erroneously detected.
On the other hand, there has been a strong demand for the provision of a method of improving the accuracy of detection of the incident position of radiation such as gamma rays in the art. In order to meet this requirement, a variety of scintillation detectors have been proposed in the art. In a first example of the scintillation detectors, a number of photomultipliers having a small end face plate and a small photocathode are arranged with high concentration. In a second example, the photomultiplier shown in FIGS. 1(A) and 1(B) is so modified that the photocathodes are further divided.
However, in the first example of the conventional scintillation detectors in which a number of small photomultipliers are arranged with high concentration, miniaturization of the photomultiplier with its characteristic maintained unchanged is limited. Furthermore, the ratio of the outside dimension of the photomultiplier to that of the photocathode is so relatively large that a part of the light from the scintillator may enter the gap between the adjacent photocathodes. That is, the light cannot be effectively utilized and accordingly it is difficult to greatly improve the accuracy of detection of the incident position of radiation.
In the second example of the conventional scintillation detectors containing the photomultiplier as shown in FIGS. 1(A) and 1(B) which is modified in such a manner that the photocathodes are further divided, the problem "light mixing" has not been solved yet, and therefore it is limited to perform the position detection with the high accuracy. Furthermore, it is necessary to provide the arrays of dynodes and the anode electrodes the numbers of which correspond to the number of division of the photocathodes, with the result that the detector is unavoidably intricate in construction and is not suitable for miniaturization.
Accordingly, an object of the invention is to provide a photomultiplier small in size and simple in construction which can be improved in the accuracy of detection of the incident position of radiation such as gamma rays.